Various types of voltage-boosting DC/DC converters are known as disclosed, for example, in Japanese Patent Laid-Open Publications (JP-A) Nos. 2003-111390, 2003-216255 and 2006-149054.
As shown in FIG. 13 hereof, the voltage-boosting DC/DC converter disclosed in JP 2006-149054 A generally comprises an input-side smoothing capacitor C1, an inductor L0, a primary winding L1, a secondary winding L2, four switching elements SW1, SW2, SW3 and SW4, and an output-side smoothing capacitor C2.
The input-side smoothing capacitor C1 is connected between a common reference terminal 11 and an input terminal 12, while the output-side smoothing capacitor C2 is connected between the common reference terminal 11 and an output terminal 13.
The primary winding L1 and the secondary winding L2 form an essential part of a transformer T1. The transformer T1 includes a single core (ferrite core, iron core or the like) F1 on which the primary winding L1 and the secondary winding L2 are wound with opposite winding directions and connected together in an oppositely-wound configuration. The winding ratio between the primary winding L1 and the secondary winding L2 is preferably 1:1.
The switching elements SW1 to SW4 are each in the form of, for example, an IGBT (Insulated Gate Bipolar Transistor) capable of conducting a high current and withstanding a high voltage. Each of the switching elements SW1 to SW4 has a collector, emitter and gate. Further, a diode D3 is connected in parallel between the collector and emitter of each of the switching elements SW1-SW4 in a forward direction from the emitter toward the collector.
The inductor L0 is connected at one end to the input terminal 12, which forms an upper terminal of the input-side smoothing capacitor C1. The other end of the inductor L0 is connected to a common terminal “c” of the primary and secondary windings L1 and L2 of the transformer T1. Two T-match circuits are connected in parallel between the other end of the inductor L0 and the output terminal 13. The parallel T-match circuits comprise a first T-match circuit including the primary winding L1 of the transformer T1 and switching elements SW1 and SW3, and a second T-match circuit including the secondary winding L2 of the transformer T1 and switching elements SW2 and SW4.
In the first T-match circuit, a point between the collector and emitter of the switching element SW1 is connected between a terminal “a” of the primary winding L1 and the common reference terminal 11, and a point between the collector and emitter of the switching element SW3 is connected between the terminal “a” and the output terminal 13. Further, in the second T-match circuit, a point between the collector and emitter of the switching element SW2 is connected between a terminal “b” of the secondary winding L2 and the common reference terminal 11, and a point between the collector and emitter of the switching element SW4 is connected between the terminal “b” and the output terminal 13. Gate signals SG1 and SG2 for controlling ON/OFF action of the two switching elements SW1 and SW2 are supplied from a control device or controller (not shown) to the respective gates G1 and G2 of the switching elements SW1 and SW2. Similarly, gate signals for controlling ON/OFF action of the remaining switching elements SW3 and SW4 are also supplied from the non-illustrated controller to the respective gates of the switching elements SW3 and SW4. In the circuit configuration shown in FIG. 13, however, the switching elements SW3 and SW4 are kept in an OFF state. In this instance, when current flows from the terminal “a” or the terminal “b” toward the output terminal 13, the diode D3 of the corresponding switching element SW3 or SW4 allows the current to flow therethrough to the output terminal 13.
FIG. 14 collectively shows the ON/OFF action of the switching elements SW1 and SW2 occurring in response to the gate signals SG1 and SG2 applied respectively thereto, waveforms of currents I1 and I2 flowing through the primary and secondary windings L1 and L2, respectively, according to the ON/OFF action of the switching elements SW1 and SW2, and the waveform of an ideal current I3 flowing through the primary and secondary windings L1 and L2.
In the voltage-boosting DC/DC converter 10 shown in FIG. 13, when the switching element SW1 is turned on, an exciting current I1 flows through the primary winding L1 of the transformer T1. As the exciting current I1 flows through the primary winding L1, an excited current (induced current) I2 is produced in the secondary winding L2 on the basis of the mutual induction action. Alternatively, when the switching element SW2 is turned on, an exciting current I2 flows through the secondary winding L2 of the transformer T1. As the exciting current I2 flows through the secondary winding L2, an excited current (induced current) is produced in the primary winding L1 on the basis of the mutual induction.
The two switching elements SW1 and SW2 are designed to perform switching operation such that, as shown in FIG. 14, the timing of switching action of one switching element occurring in response to one of the two gate signals of different phases is the same as the timing of switching action of another switching element. The switching actions of the switching elements SW1 and SW2 have the same time period A, B corresponding to unit waveforms of the currents I1 and I2 flowing through the primary and secondary windings L1 and L2. The switching elements SW1 and SW2 have the same ON time C, D.
In the conventional DC/DC converter 10, the current flowing through the primary winding L1 of the transformer T1 and the current flowing through the secondary winding L2 ideally have a waveform (ideal current waveform) I3, which is continuous in regions 15 occurring repeatedly at switching of mutual energization of the primary and secondary windings L1 and L2. In practice, however, due to a difference in inductance of the primary and secondary windings L1 and L2, or a difference in ON/OFF characteristic of the switching elements SW1 and SW2, switching of mutual energization of the primary and secondary windings L1 and L2 produces a current difference, which creates an abrupt change (or stepped portion) in each of regions 16 of the waveforms of the currents I1 and I2. The region 15 in the ideal current waveform I3 and the region 16 of the waveforms of actual currents I1 and I2 are shown on enlarged scale in FIGS. 15A and 15B, respectively.
The abrupt change (stepped portion) 16 occurring in the regions 16 of the waveforms of the currents I1 and I2 increases current ripple in the transformer T1, which may sometimes be 5 or more times as large as the ideal current waveform I3. With this increase in the current ripple, iron loss of the transformer T1 increases, resulting in undue temperature rise and efficiency reduction of the transformer T1. In some cases, the transformer T1 undergoes magnetic saturation. At the stepped portion 16a (FIG. 15B) of the waveforms of the currents I1 and I2, a harmonic component is involved, which causes the transformer T1 to generate unpleasant vibration noises. Further, an increased current ripple gives a negative influence on currents flowing through the switching elements SW1 and SW2 so that the diodes D3 associated with the switching elements SW1 to SW4 are subjected to an increased peak current. This may require use of switching elements of higher capacity capable of providing an increased current rating (withstanding current).
With the foregoing drawbacks in view, the present invention seeks to provide a phase control device and a phase control program, which are capable of suppressing creation of an unnecessary abrupt change (stepped portion) in waveforms of currents flowing in the primary and secondary windings of a transformer of a DC/DC converter.